text:
What does the airport fuel market look like?
The single biggest cost for many airlines is fuel. Given this, I'm sure they work hard to reduce the cost they pay per gallon of fuel as much as possible.
How does that translate to the way fuel is bought at major airports? I know there's a fuel truck that comes and fills up the planes. Does each airport partner with one company to provide fuel? Are the multiple providers on each airport? Does the airport itself sell fuel? Is fuel lumped into the landing fee?

Airlines are big users of currency and fuel options to hedge against fuel price and currency fluctuations. In the end, the lower risk of price changes is more important than getting the lowest possible price, but risking to be bankrupted by a price change.

@PeterKämpf that sounds like some very fancy economics! Does that imply the fuel is physically pumped by airline trucks but that fuel will have been sold and bought on some sort of an open market?

There is no commercial connection between the options and the actual fuel, outside of the general market price. Airlines trade with options which help to compensate them for the fuel price fluctuations. The fuel is still bought in the traditional way, but the airline needs not to worry about the market price of oil. Some are still hit, however, if they do not cover 100% of their needs or if the price changes are outside of their expectations and the range which the options cover.

@PeterKämpf what's "the traditional way"?

Get a bill from the fuel truck, or the airport, or give the oil company access to your bank accounts.

The fuel is not always the biggest cost for the airline, for long haul it is true, but for short range depends strongly on the route. However, it is true that is always a significant cost to be reduced. I would also to stress on point that has been mentioned but not expanded. Fuel is indexed to a dolar reference and many airlines are earning incomes in other currencies. Protection is dual, fuel and currency fluctuations using options and swaps as financial mechanisms.

@TrebiaProject. That's why I said "many airlines"

Large airlines have standing, negotiated contracts with major airports to buy fuel (and almost always also other services like cleaning, catering, etc).  Some times these are fixed-price contracts, sometimes they float in relationship to the spot price of jet fuel, sometimes they are a hybrid.  You can think of these as any other complex financial relationships: almost any way you can imagine a contract to be drawn up, someone has likely tried it.
Airlines' operations desks also constantly monitor the price of fuel at different airports and different times - and in some cases airplanes carry enough fuel to make a return trip without refueling because it ends up being cheaper to carry the fuel rather than purchase it at the next stop.
For general aviation, the airport FBO (fixed base operation) generally has one and sometimes multiple fuel vendors on-site, that generally sell at a fixed price -- very much like a gas station for cars.  Obviously, airports that have more than one fuel vendor generally have more competitive prices, but even for single-vendor operations they can't make the fuel costs TOO high, or pilots will purchase just enough fuel to take off and go somewhere that gas is cheaper.

Not only general aviation, but many small airlines also purchase fuel at FBO and the captain simply pays for it with company credit card.


text:
Why do pilots' masks have more oxygen?
When a commercial passenger airliner suffers a rapid decompression, it is critical that everybody on board gets oxygen quickly. In the passenger rows, oxygen masks connected to chemical oxygen masks will drop. In the cabin, the flight crew will pull out their oxygen masks, which are connected to pressurized oxygen canisters. The chemical generators will last for 10-15 minutes, but apparently the flight crew's oxygen supply will last for a couple of hours.
Why would the flight crew's supply last for so much longer? If the plane isn't at a breathable altitude after 30 minutes, it seems like it'd probably be unrecoverable, and the passengers will be in irreversible comas anyway.

Basically, if the passengers all pass out, it's not a huge deal.  They will likely recover.  If the pilots pass out, the plane is likely to crash - which is exactly what is thought to have happened with Helios Airways Flight 522.

@KeeganMcCarthy as I mentioned in my post, the passengers can only recover up to a point. After about 30 minutes at cruising altitude, most will be in an irreversible coma. If the Helios 522 pilots had put their masks on, they would have had ample time to descend to a breathable altitude.

I know that there was a lot more to Helios 522, but I was just giving an example of what can happen if a pilot doesn't have O2, vs passengers not having it.  Has a plane ever been at cruising altitude for more than 30min without O2?  I'm sure it CAN happen, but it is very unlikely.

Helios 522, the pilots mistook a depressurization warning for a takeoff configuration warning. The master warning/caution went off (not sure which) after the oxygen masks dropped, but they thought it was for overheating systems. The pressurization was on manual after a test instead of auto and the pilots didn't know.

@ptgflyer: Master caution.  737s don't have a master warning.

The pilots also need to use oxygen in case of smoke, fumes, or fire in the aircraft.  Thus, they need enough oxygen to be able to get down and land in a fire scenario, which is much more than a decompression!
Fire procedures have the crew on 100% oxygen at positive pressure to prevent them from breathing in smoke and fumes, while in a decompression, their masks mix oxygen with cabin air, in much the same way the cabin masks do.  This is also why the cabin masks are not used in smoke/fumes scenarios.

In a fume event, the pilots may need to keep using oxygen until landing. There are routes with points almost 6 hours from nearest usable airport, so the aircraft that fly them have to have corresponding amount of oxygen for the flight crew.

Limited space and weight come into play.  As well as the fact that sufficient air to keep one alive is not enough for peak performance.  The masks for passengers are enough to keep the passengers alive during the descent to a survivable air pressure.  Once there, the pilots will still want more oxygen so they can better deal with the emergency (and landing preparations).

Also ... AFAIK Pilots are required to wear an oxygen mask when alone in the cockpit (e.g. when co-pilot steps out of the cockpit for a moment) - So they probably use up oxygen routinely. Also the plane can survive passengers being woozy and unable to concentrate, by contrast, the plane is in jeopardy if pilots are anoxic.

Also, during the cockpit preflight, the cockpit crew dons their oxygen mask to check for oxygen flow, which uses a little of the supply each time. In addition, some use oxygen as a "pick me up". When I saw that happening, I usually remarked, "Hard night last night?"

@RedGrittyBrick Isn't the requirement for that situation just that the mask automatically turns on flow in the event of a decompression? I don't think the mask actually has to be drawing from supplemental oxygen unless cabin pressure actually drops below a certain level.

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text:
Why we don't see the "SkyRider" seats in action?
In 2010 nearly all newspaper reported on the new invention of the company Aviointeriors.
They created new airplane seats called "SkyRider" which you can "stand" in. This should increase Airlines profitability.
I'm not a big fan of the SkyRider seats but who can I thank, or why don't we see them in Action?

Because they are awful, horrible things spawned by the minions of hades. Or the airlines, not sure which is worse.

If they really wanted to lay people flat they should just give everyone their own little bed.  We'd all be more comfy and they could fit more people in.

Was the announcement by any chance on April 1? It's hard to imagine that's a serious proposal.

@JayCarr Skyrider only counts as lying flat if the plane is in a vertical climb.

@DavidRicherby Well, yes, that would, I'm saying they could put beds in planes and possible pack more people in as a result.  Just go with 3 high bunks, it'd be fun! ...or woefully impractical, take your pick.

@JayCarr Too many people would start having sex.  Being in a position roughly equivalent to being duct-taped to a tree keeps everyone equal parts miserable and well-behaved.

There are a lot of disadvantages, even if the possibility of lowering costs and increasing profits is a huge advantage for the airlines. This article discussed several issues from a passenger perspective:

Limited space in general (e.g. to use a laptop)
Face too close to the video screen in front of you (if there is one)
Doesn't fit tall, large, short or young people well (and I guess that older people might not be strong enough to use it)
Very limited space for carry-on luggage (because you lose space both under the seats and in the overhead bins)
Passenger resistance to the basic idea

The author's conclusion was that the seat isn't actually uncomfortable, just cramped, but he was very sceptical that passengers would be willing to use it.
Another article added some concerns from the airlines' point of view:

Could make cabin evacuations more difficult (and certification costs in general would be high for a completely new seating concept)
Difficult to market successfully
The manufacturer intended the seats to be used in a new budget fare class, not to replace current economy class seats (this adds more complexity to everything: aircraft layout and maintenance, marketing, booking systems etc.)

Both articles (and others) concluded that there's no fundamental technical or regulatory barrier to introducing them, the main problem would be convincing passengers to accept them. Even Ryanair - which is notorious for cutting costs wherever possible - rejected them although Wikipedia says that both Ryanair and Airbus have proposed an alternative but very similar concept in the past.
If the airlines really wanted to introduce this seating arrangement, I would expect them to keep the idea in the news as much as possible and generally work on convincing people that the seats are comfortable and it's a great way for them to get even cheaper tickets. Since they haven't done that (if they have, I haven't noticed) it seems unlikely that the airlines are actively working on them.

Well, you use the overhead lockers completely, because you need at least 2.10 meters height for such a seat, while now there's surely less than 1.70 under the lockers since I'm far away from being able to stand there.

There are 2 major problems with skyriders

They need to be certified, just designing and showcasing them isn't enough to allow you to actually put them in planes. All that media buzz was to round up investors to get them through the certification process and get ready for mass production.
They are not one size fits all, as your second pic shows; overweight passengers will have trouble using the seats, same with children.

Then there is the "bad press" that got around. They would turn economy class even more into the sardine can than it is now.

These are both important problems, but I think the most important problem is that everyone would hate them. It would have to be a really, really short flight and a really, really cheap ticket for most people to consider purchasing such a seat. Even introducing them as a separate class would likely harm an airline's reputation with customers.

getting them on buses first would help to make people adjust to them

@ratchetfreak Most commercial flights involve being in the plane for at least an hour. Most bus journeys involve being in the bus for much less than an hour.

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text:
Which Airline don't use the Flight Number 191 anymore because of bad luck?
This question came up while reading an article about mysterious plane incidents. It told that some airlines do not use the flight number 191 because of bad luck.

In fact, there have been so many catastrophes that, much like hotel
  owners who refuse to have a 13th floor, some superstitious airlines
  have completely done away with the number “191.”

Does somebody know some Airlines who are not using this flight number
for that reason?

Well, you can be pretty sure that Delta and American no longer use that flight number, as it is pretty standard for airline to retire a flight number of an accident flight.
Beyond that, I'm sure there are a multitude of carriers still using Flight 191.
For example, Air India 191 is in-flight at the time of writing . Air Canada 191 is scheduled


text:
Why is the A400M APU on the side of the fuselage instead of the back?
As you can see in this picture the APU of the A400M is on the side under the right wing, including the RAM-Air-Turbine.

Why is the A400M APU on the side of the fuselage instead of the back?
Why is the RAM-Air-Turbine not on the bottom of the fuselage?


text:
Why do Turkey and UAE limit the age of aircraft flying over their territory?
Why do the civil aviation authorities of Turkey (15 years civil, 25 years cargo) and United Arab Emirates (20 years) limit the age of aircraft flying over their territory? I think if an old plane receives good maintenance and doesn't have too many cycles, it's still a safe plane.
For reference, see this news article of a leased Kabo 747-200 that is being refused clearance by the UAE because it is older than 20 years.(German/English)

Also interesting in the article linked: "As quoted above, the provision of article 19(i) does not refer to the date of manufacture for aircrafts already registered in Turkey under any circumstances whatsoever. It only refers to the first registration date to the Turkish Civil Aircraft Registry that makes difficult to realize the aim of the provision if the interest is airworthiness itself."

Pure speculation here, but I wonder if it's not co-incidence that the refused plane cited in the article is a charter plane for the Hajj, and both UAE and Turkey are on popular routes for Hajj pilgrims. I wonder if there's a history of unscrupulous or under-qualified operators chartering old, poorly maintained planes for lucrative Hajj trips, which then needed to make disruptive emergency landings?

Apparently they easily turn the blind eye on it when it suits them. A lot of cargo is supplied to UAE in An-124; I've seen up to 3 of them parked in Abu Dhabi at a time. All of them are significantly older than 20 years.

Sounds like a market protectionist move by the nations. i.e. The airlines of UAE and Turkey are relatively new. At least their current expansion to a scale that is competitive with the major international airlines is relatively recent. Their airports have also undergone fairly recent expansions to capitalize on their geographical location as hubs between N. America / EU to Asia-Pacific nations.
I suppose they give their own airlines a competitive advantage by restricting older aircraft. The US / European carriers can only allocate the fraction of their fleet that is new to flights to Turkey / UAE which constrains their activities.
The average fleet age of some airlines in 2013:

Delta  16 years
United  13 years
American + US 13 years
Lufthansa 13 years

Now compare with

Turkish  8 years
Emirates 6 years

Basically, international trade treaties have made it difficult for nations to overtly engage in protectionist measures. So bureaucrats figure out indirect means under the guise of technology, quality etc. that are ostensibly  not favoring domestic industry but are effectively a discriminatory measure.
Take the numbers with a pinch of salt because airline fleets are continuously upgraded so the average age of a fleet can change pretty rapidly.
Caveat: The average fleet age may not be representative of the equipment flying international sectors.
PS. I don't think the regulation has much to do with real safety because safety is concerned with many other factors outside of aircraft age. The safety aspects are better regulated by adopting the FAA safety watch-list based on national assessments.
At best, this sounds safety theater. Not safety.

I don't believe there's been a stated reason, but we can surmise several
Newer aircraft have newer, more efficient engines meaning typically lower noise and less pollution.
Newer aircraft are typically better maintained. Obviously this isn't always true, and some older aircraft can be very well maintained, but it's easier to just put a blanket cut-off policy in place, rather than waste time on a case-by-case basis. It also gives the perception to the public that a safety policy has been put in place.
Even if they aren't better maintained, newer aircraft are less likely to have developed faults. Again, there is no guarantee, but it provides a lower risk profile.
And as stated in another answer, there is possibly a degree of protectionism - local airlines in that part of the world tend to be fairly young with modern fleets, whereas some of the larger carriers elsewhere have older fleets. I'm not convinced this would be an effective tactic, however, as the likes of Lufthansa, British Airways etc have enough new planes to service routes to the UAE etc
And finally, older aircraft tend to be snapped up by less.... Conscientious... Airlines. Age limits are a good way to force these types of low-cost, potentially 'iffy' airlines out of contention if you don't want them flying your people in and out of your airlines.

The idea that new aircraft are "less likely to have developed faults" is demonstrably false (Square windows seemed like a great idea, as did lithium battery cells)

I don't disagree, but whether an idea is true or false doesn't generally prevent policy makers from basing decisions on it...

Stop hanging out with the United States Congress Jon! :)

Isn't a new (coming out of the factory) aircraft of the same type/model safer than older ones (because the new one has less wear and tear) ? Are there any data that refute that?

Actually, the older one is more safe. It has been burnt-in - that is, any early-life failure modes have been shown to be absent (it's still flying) or, if they failed non-catastrophically, have been fixed. Check out Reliability Theory for more info.

Again I don't disagree, but the public perception is that newer = better, despite most evidence suggesting there's no real difference

Age limits are a good way to force these types of low-cost: If it were an age limit, it would be ok, but it isn't... It's the age of the registration to the Turkish Civil Aircraft Registry. This makes possible to buy an old aircraft from a foreign owner, and register it in Turkey. Or I've missed something. In addition it tends to null the value after 15 years, hence manufacturers will be requested to sell a/c that don't last more, at a lower price. Not in favor of safety.

That's interesting, although my answer doesn't discuss the Turkish situation, only the UAE (which I believe is truly from date of manufacture). That seems very strange, considering Article 57 of the Turkish Civil Aviation Act confirms that "Date of manufacture" is part of the required registration, but does appear to be true.

@voretaq7: Lithium-ion batteries are a great idea; the 787 batteries just had the teething troubles that occur with everything the first time it's included in an aircraft.


text:
Why was this Twin-Otter Take-Off unsuccessful?
Watching this video I was asking myself, what was the error or cause of this unsuccessful take-off?

the video has some comments under it, which seem to explain what was wrong in this take off

Simple: the pilots did not do their pre-flight calculations.

Comments under the video say clearly:
According to Transportation Safety Board (Canada) report # AO7W0128,

Take-off weight did not meet performance capabilities of the Aircraft.
Calculations to clear 50ft obstacle were not performed.
Southeast end of runway, not clearly marked.
Take-off was attempted in an up slope direction with a slight tailwind which increased the necessary distance to clear a 50ft obstacle.
Center of gravity (CG) calculations were off and found to be too far forward.

You might add that the pilot tried to rotate too early (twice), instead of letting speed build up to proper rotation speed.

In the official report, I do not see a mention of the pilot rotating too early.

It can be seen in the video. An then the plane lifts one wing, while the other is stalled - because the pilot tried to rotate with inadequate speed.

One is at about 1:44, where he rotates far enough to lift the front wheel clean up


text:
What is the difference between MOCA and OCA?
Can someone explain me the difference between minimum obstacle clearance altitude and obstacle clearance altitude? I read definitions in Doc 8168 but i don't really see the difference. I would be grateful if someone explain where which type of altitude is used.
From ICAO Doc 8168:

Minimum obstacle clearance altitude (MOCA). The minimum altitude for a defined segment that provides the required obstacle clearance.

Obstacle clearance altitude (OCA) or obstacle clearance height (OCH). The lowest altitude or the lowest height above the elevation of the relevant runway threshold or the aerodrome elevation as applicable, used in establishing compliance with appropriate obstacle clearance criteria.

It seems clear to me: MOCA is for airway segments (i.e. enroute procedures), OCA/OCH is for airports (i.e. terminal procedures).

Closely reading the definitions in the OP,

the MOCA exists with reference to a defined segment, while
the OCA or OCH exists with reference to a specified runway threshold or a specified aerodrome.

In other words, one applies to a line segment, while the other applies to a point.  "This" segment has its particular MOCA, while the OCA/OCH would apply to whatever procedures are being designed for "this" runway or airport.

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text:
What is the purpose of the fin sweep on small piston airplanes?
Why some small piston airplanes, like Cessna 152/172, Tecnam P2008 and many more, have the vertical stabilizer with a sweepback? I don't think that those airplanes may encounter compressibility effects, so what is the reason?

Because it looks cool? (Warning: TVTropes)

Two benefits to fin sweep on a type like a 152 or 172:
Marketing, and aerodyanamics.
The airplane looks "cooler", "going fast when it's tied down" with the sharply swept fin; this makes it easier to sell to casual pilots and those who'll rent the plane to casual pilots.
However, there's also another advantage.  The sweep moves the tail area to the rear without adding dead weight of structure far out in the tail.  Thus rudder authority and yaw stability are gained with little increase in overall weight, compared to the previous generations of the same models in the Cessna line, for instance (look at a 180 from 1960 for comparison to a modern 182).

The further back the fin and rudder are, the more effective they are. But the longer the fuselage is, the heavier it gets. Sweeping the fin back is a way to move its centre of pressure aft and increase its effectiveness, without lengthening the fuselage.

But doesn't a swept fin has lower lift per area if comaperd to a straight fin? Is the net result better (longer arm to the fin CP vs greater lift on a fin)?

@Konrad Yes. But aircraft design is all about compromise to achieve the optimal solution. A moderate sweepback adds more leverage than it takes away. But sweep back too far and it takes away more than it gives back. So one aims for the sweet spot.


text:
Why does Vr increase with lower air density and decrease with higher air density?
Can someone explain why Vr increase with lower air density and decrease with higher air density? I read such a phrase in my ATPL(A) performance textbook, but without further explanation. As far as I know Vr is an indicated airspeed, so it should be independent from air density.


text:
Why only the outboard ("low speed") aileron drops when there is no hydraulic pressure on the 747-8?
As the title says, why only the outboard ("low speed") aileron drops when there is no hydraulic pressure on the 748?

(Full resolution)

Please could you be a bit more precise in your question, what phase? Are you considering the aileron DROOP function that is the flaperon  Function of ailerons? What do you mean no hydraulic? Just for this aileron or more than that. However many servo controls become floating in absence of pressure. On ground it drops by its weight. In flight not to twist the wing, it will float.

@user40476 On the ground, without hydraulic pressure, only the outboard aileron drops due to its weight. I don't talk about the droop function. I think the answer lies in the fact that the inboard aileron is actually a flaperon

At high speed flaps up a 28 volts signal locks out the outer ailerons only, may be what happens on ground, hydraulic off is just a consequence of the servo control design.

If the servo control has an idle circuit they will droop unpressurized.  If no idle circuit they are hydraulically locked when there is no movement of the servo control out of null and they will stay rigid while unpressurized.

@John K , it looks to be locked it needs the 28 volts signal, the absence of this signal is probably the reason of the droop

The Boeing 747-8 (unlike the 777) is not a full fly-by-wire aircraft, but some fly-by-wire controls have been added to the original 747 design. The inboard ailerons are mechanically connected to the flight controls, while the outboard ailerons are not. This allows them to droop without hydraulic power.
From the Boeing 747-8 FCOM (9.20.6 Flight Controls - System Description):

Roll Control
Inboard ailerons are mechanically controlled and hydraulically powered. Outboard ailerons and all spoilers are electronically controlled and hydraulically powered.
Aileron and Spoiler Roll Control
Rotating either control wheel positions ailerons and spoilers. Above approximately 258 knots/.53 Mach, the outboard ailerons are commanded to the neutral position to prevent overcontrolling. The outboard ailerons become operational at lower airspeeds. Inboard aileron deflection is limited when flaps are extended beyond 10.
To improve takeoff performance and reduce noise, the outboard aileron is deflected down for flap detent positions 10 and 20. The downward deflection is removed if the angle of attack is above stick shaker. The inboard aileron is deflected down when the flaps are selected to 10 or greater.
All spoiler panels operate with the ailerons. Spoiler panels are deflected up or down based on Speedbrake lever and control wheel inputs.

The following image from modernairliners.com shows all new features of the 747-8 wing including the Aileron droop.


text:
What is this object on the tail of an F-16?

Source: skytamer.com
What is the object on the tail of this F-16 (next to the serial number)?

Must be where the jump starter cable goes ;-)

Chemtrail Output Valve (portside)

@Richard: close. That's the starboard side of the ship.


text:
What should I build my ultralight out of?
I have always wanted to fly and I have taken some ground school but I don’t have enough money to finish my ppl and get a plane so I think ultralights are the next best thing, it is just what should I build it out of. I have seen many people use wood, aluminum, or steel. What would be the best bet for a new builder that has a lot of understanding how a plane works

Welcome to Aviation.SE!  While you might receive some good answers, it is also possible that your question will be closed.  If you edit your question to clarify what research you have done so far and what you are looking for, it is less likely to be closed.

There is a young man who built an ultralight out of foam insulation. https://hackaday.com/2017/08/14/building-an-ultralight-out-of-foam-in-a-basement/ Since this article came out, he has successfully flown it, I saw it on a TV program.  Google his name, or "foam ultralight" for more hits.  He's a youtuber apparently, more hits in this article http://inspire.eaa.org/2017/11/21/airventure-2017-inspires-homebuilt-foam-board-ultralight/

Have you tried your local glider club. I have nothing against ultralights. If your end goal is to get your private pilots certificate, it may be more cost effective to spend your time and money at a club, renting aircraft, than to buy/build an ultralight. You can build the ultralight later if you want. But, as of 31 Jan 2012, ultralight experience can not be logged as aeronautical experience towards an FAA license per Part 61.52.

Cheapest solution: Take ultralight training, then buy a second hand ultralight, bringing along an expert to evaluate it.  Kit built ultralights depreciate heavily in the first 10 years.  Building from a kit will cost more than buying a used puddle jumper aircraft like a Champ.  You'll have 20-30k into it and it will be worth 5-10k in 10 years.

Possibly discarded face masks?

Airplane parts.

@CrossRoads - I think the name you're looking for is Peter Sripol - https://www.youtube.com/watch?v=fqdtz3tJkbY

The best solution would be to buy a kit where an engineer has worked out the materials, fasteners, etc. into a proven design that likely won't kill you from mechanical failure assuming correct assembly and proper flight control manipulation by you, etc.

You left out "probably" -- no design is proof against mechanical failures.

That's why a lot of builders do a static load test using sandbags or equivalent on the airframe before cover.


text:
How do water bomber pilots deal with the changing forces and mass when scooping water?
How does a pilot flying the scooping maneuver in a CL415 firefighting plane deal with the changing forces from taking on such a large mass of water (nearly 50% of the plane's own mass) as quickly as these planes do?
Some basic math indicates that the scooping maneuver would generate $F=\dot{m}v = 18.4\ \mathrm{kN}$ of additional backwards force just from accelerating the $\dot{m} = \frac{\Delta m}{\Delta t} = \frac{6140\ \mathrm{kg}}{12\ \mathrm{s}} = 512\ \mathrm{kg/s}$ of scooped water up to the plane's $v = 36\ \mathrm{m/s}$ scooping speed.
Or using a different calculation, with no additional thrust added to compensate, the plane would slow from 70 to about 50 knots from taking on the water. Plus, there's other forces generated here too — a downward force from accelerating the water vertically to lift it into the plane's tank, probably a pitch-down torque from the scoop placement, the additional weight from the changing mass, and also just the hydrodynamic drag forces that seaplanes also deal with landing or taking off.
Some of the control inputs seem clear, like needing to throttle up and probably pitch up slightly, but how does the pilot determine the adjustments to make here, and how do they coordinate the timing for making these adjustments?
Also, how does adjusting trim for the empty vs full states work? I'm imagining that there'd be some way to mark down separate trim settings for empty vs full and then use that to quickly switch to the appropriate trim after scooping or dumping the load, but is this done manually or is this automated in some way?

I don't know how to fly a water bomber, but I can put your numbers in perspective: 18.4kN at 36m/s is 'only' about 900hp, which is less than 20% of the total installed power of the CL415.

The 13,500 lbs of water that is scooped is very near the center of gravity, and is is only about 1/3 of the 47,000 lbs total weight of the aircraft. The trim changes are handled by “seat of the pants” manual adjustments.

@Sanchises Sure, and I have no doubt that the plane has enough power to handle it. This is more about how the pilot coordinates applying that power while transitioning from the scoop approach to making contact with the water.

I have experience on seaplanes, although not flying boats, and used to teach float ratings. The techniques for flying boats and floatplanes are more or less the same except for the differences due to the tip floats.
When you fly floats, you learn to apply up elevator to compensate for a pitch down from the drag of the floats at water contact (not too aggressively, or you pop back into the air; just enough).  In fact, when landing on glassy water, which is normally done with a special procedure where you fly on very gently because you don't know your precise height, the pitch over is the signal that you have touched down.
Same with a flying boat.  When you have scoops sticking down, it just makes the pitch over and deceleration stronger, and you have to apply more up elevator to compensate, but the timing and application is exactly the same as a normal landing, just more.  On some waterbombers the intake probes are extended after stabilizing on the step at touchdown, so there two separate pitch reactions, but again, you are just responding to an action you are expecting.
You normally land seaplanes with power on until you've settled on the step after water contact, so the procedure if you're scooping water is to check the pitch over and add more power, enough to keep the plane from decelerating too much, and just hold that until you retract the scoops. Whatever the pitching tendency is from the scoops going down and up, you just learn to counteract it with control inputs intuitively, and similarly with power.

I used to fly PBY water bombers in the 1980s. After touchdown on the water, we would add a little to power to stabilize the aircraft on the step, and then lower the single probe, while adding full power. When the tanks were full we would raise the probe and let the aircraft accelerate and takeoff.

My boss at the bush air service I flew at in the early 90s was an ex Canso (commonwealth name for the PBY) waterbomber pilot and he said it was like "waltzing with a gorilla" with the heavy controls, and his copilot did the job of providing aileron boost during heavy maneuvering. Were they that bad?  I was pretty sure the 415s touch down with the probes out, but now I have to go check.


text:
How does rudder lock occur?
Citing Wikipedia article on the Vertical Stabiliser:

Rudder lock occurs when the force on a deflected rudder (in a steady
  sideslip) suddenly reverses as the vertical stabilizer stalls. This
  may leave the rudder stuck at full deflection with the pilot unable to
  recenter it.

But how does this phenomenon occur from an aerodynamics point of view? What does it mean that the force reverses on the rudder? Is pedal force meant here? Furthermore, how can the pilot recover from rudder lock?

Normally, the airflow over the vertical stabilizer is from front to back, which means it hits the front face of the rudder, and acts to centre it. But the vertical stabilizer is actually like a small wing. In a sideslip, the airflow comes more from the side. If you're slipping tail-right, using left rudder, the airflow comes from the right. When the vertical stabilizer stalls, the airflow is no longer acting to centre the rudder: it's just pushing against it, and pushing it away from the centre.

Is it possible to recover?

@Steve I'm not pilot but logically to break the rudder lock the force on the rudder generated by the airflow must decrease to the point that the force that can be exerted on the rudder by the controls is greater. If the airflow can be changed with the rudder locked, then it may be possible to recover. if the rudder lock reinforces the motion of the plane that is creating the locking airflow, then probably not.

Rudder lock requires a mechanical linkage between the rudder pedals and the rudder. With no force on the pedals, the rudder will then weathervane (float) into an equilibrium position that depends on the sideslip angle and the hinge moment coefficients of that particular rudder.
With enough sideslip angle the rudder will hit its deflection limits and stop to move further. If the sideslip angle still increases, the difference between the maximum deflection and the theoretical floating angle (if there were no mechanical stops) will determine the aerodynamic force by which the rudder is pressed into the stops.
At this point the rudder will exhibit fully separated flow on its leeward side, but that only changes the hinge moment derivatives and the resulting floating angle. Normally, the ratio between the change in sideslip angle and the resulting change in floating angle is less that one for small angles but increases above one for larger angles. This means that the rudder will experience a steep hinge moment increase for small increases of the sideslip angle once it has reached its maximum deflection.
How far the pilot is able to overcome those forces and bring the rudder back to neutral depends on the size of the rudder and the dynamic pressure. Ideally, he/she would never allow the rudder to float, which also should ensure that sideslip angles stay low. However, in some maneuvers like a sideslip you want high sideslip angles, and there a locked rudder is quite normal.
When I was flying the Schempp-Hirth Discus, I could either float gently into a sideslip with moderate angles and no rudder lock, or violently swing the aircraft into the sideslip, which resulted in a higher trimmed sideslip angle and a locked rudder. However, rudder forces in a glider are low and ending this condition was trivially easy.
I also witnessed a crash in my career when a test flight with a marginally stable aircraft went wrong. The aircraft was flying at Mach 0.7 and had a fully mechanical rudder linkage. The pilot had taken his feet off the pedals and the aircraft went into a sideslip when he did aileron doublets. Only when the rudder deflection had reached 10° did he try to correct that condition (at this point the sideslip angle was also about 10°), but the deteriorating directional stability at higher sideslip pushed the aircraft into a maximum 27° of sideslip. Since the maximum rudder deflection was only 20°, the aircraft regained some stability with the locked rudder at the maximum sideslip angle and ended up in a yaw osciallation between 17° and 27° of sideslip. The pedal forces involved were too high to be overcome. Unfortunately, the tail was a T-tail and produced a strong pitch-down moment with those higher sideslip angles, and also the stick forces were too high to correct that. In the end, the aircraft dove into the ground.

About 1.5 paragaphs in: "This is Peter Kämpf, isn't it? scroll down Yep."

Did the accident you referred to involve the Grob SPn?

@mezzanaccio No, it was the Ranger 2000.

@PeterKämpf What's floating angle? Is it the angle subtended by the rudder from the centerline?

@Auberron Floating angle occurs in a reversible control system and is the flap angle without control input. Say you leave your feet on the floor and the airplane begins to sideslip, maybe from an aileron input. The rudder will float from the center position in (non-linear) proportion to the sideslip angle. So yes, it is the angle subtended by the rudder from the centerline, in the special case of no force applied to the pedals.


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Can the design for neutral stability reduce trim drag?
If one were to design an aircraft to have neutral longitudinal stability would this reduce the required horizontal stabiliser surface and hence trim drag?

With a trailing stabilizer configuration the stabilizer would not produce negative lift which would reduce trim drag.

Terminology first: Trim drag is the drag component added by adjusting the incidence of the horizontal tail rsp. the deflection angle of the elevator for trim. The increased parasitic drag of a larger tail surface itself is already part of the aircraft's zero-lift drag.
Sizing criteria for tail surfaces
The horizontal stabilizer surface must be dimensioned with these criteria in mind:

Sufficient static longitudinal stability. To achieve this, the tail volume (tail area times lever arm) must be high enough. Use similar aircraft and copy what has been found to work well.
Sufficient pitch damping. This is driven by tail size times the square of the tail lever arm.
Sufficient control power at the most forward cg location over the whole speed range.
Sufficient control power at the most forward cg location over all load factors (including pitch damping effects).
Sufficient control power when flaring in ground effect (which reduces elevator effectivity)
For T-tail configurations: Sufficient control power over all sideslip angles. Sideslip produces a strong pitch-down moment in T-tail configurations.
For gliders: Sufficient compensation for the force of the tow line, both at the beginning of a winch launch (tow line horizontal) and at the end (tow line vertical).

The other criteria (upper and lower limits on stick force gradients over speed and load factor) can be tailored with elevator chord, so the horizontal tail surface is not directly affected.
In the end your horizontal tail will be as large as you want your cg range to be wide. Operators like wide cg ranges, so the small drag penalty of a bigger tail surface might be worth it.
Wing airfoil
Also, wing airfoils with a high pitching moment (think rear loading, i.e. high camber in the rear part) will need a bigger tail surface due to the larger shift of the center of pressure over speed, but have less drag and a higher maximum lift than comparable, old-fashioned airfoils (think NACA 4-digit range), so the wing can be smaller and has less drag, which easily compensates for the increased tail surface.
Do not optimize each surface separately
If your wing has already an optimized lift distribution, adding more lift in the same Treffz plane will spoil the end result. In the end, it does not make much of a difference at which position in streamwise direction lift is created, only the sum of it counts. Therefore, it might be better to have as little tail loading as possible in the design point.
If you take the mass of the wing structure in consideration, your optimum lift distribution over the wingspan will change to a more triangular shape, and now it might even make sense to create a small downforce on the tail to reduce the downwash angle in the center section. A lighter wing will require less lift and help to reduce drag.

Think of it this way: The triangular lift distribution produces a higher downwash angle at the tail, and any downforce there would actually point slightly forward, creating induced thrust. Effectively, it helps to equalize downwash over span in the Treffz plane of wing and tail.

Correct me if I'm wrong please, according to the picture in your answer. With such downwash trim drag is zero, and even apart from drag, induced thrust is generated.

@Konrad Yes, but at the price of more induced drag on the wing. Overall, drag will be higher. See it this way: The tail helps to recoup some of the losses suffered by the wing from a suboptimal lift distribution.


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What drives the "Engine Hours" on the G1000?
I fly a 2009 C182T with CAP. We record the "TACH" time from the Engine/System page where ENG HRS appear. On the steam gauge Cessnas I fly TACH time is read off of the tachometer and generally understood to record the time that the engine is run above approximately 1000 RPM which produces the requisite oil pressure. The POH for the 182T only describes that the HOBBS requires oil pressure of 20 PSI to run. However, I have not been able to find what sensor or stimulus runs the ENG HRS. Anyone know where this information can be found?

Your understanding of how tach hours work is not correct, you are conflating some elements of Hobbs time and tach time with each other.
Tach "time" is not electrical, it is driven solely by the rotation of the engine.  In my 182, the tach "odometer" is calibrated so that 1 "hour" on the meter means 144,000 rotations of the engine...  That's 2,400 RPM * 60 minutes = 144,000 RPH.  If the engine makes 72,000 rotations, the engine hours meter will read  0.5.  I could be turning the prop by hand and take two full days to spin the prop 72,000 times, the tach will still read 0.5.
I often cruise at about 2,300 RPM, and thus after a one hour flight, my "tach time" will be 2,300/2,400 or 0.96.  Fly for an hour at 2,000 RPM, and tach time will be 2,000/2,400 or 0.83.
So "engine hours" aren't really hours at all, but rather "equivalent hours if the engine was running at typical cruise RPM".  It's most likely based on your POH's published cruise numbers from Section 5.  It's intended to be an accurate reflection of engine use, and thus is the basis for engine maintenance intervals like TBO, SBs, and oil changes. The Hobbs is more oriented to an accurate reflection of aircraft block time, more useful for rental rates, etc.

Are you sure it's calibrated 1/1 to redline RPM and not rated cruise RPM?

I suspect you are exactly right.  I'll clarify the answer, thanks!

One reason I liked to cruise a bit oversquare, like 22"-24"/2100 rpm. Slows down the tach hours per hour in the air, reduces overall wear and tear, and you can't hurt a normally aspirated engine doing it as long as you aren't in a yellow arc or it's running rough.

23 over 2100 at 6,000ft gets me 12.6 GPH, and works out to be my singular lowest cost per NM, it's definitely a sweet spot.  I'm still traveling at 149 Kts so I'm giving up 6 Kts.  I'm getting 96% of the speed in return for 87.5% of the engine wear and 91% of the fuel burn.

Great! Thank you for the explanation. I frequently hear people describe HOBBS time as run when electrical power is turned on and know this to be incorrect for the planes I fly at least. Can you reference any documentation that supports the tach time calculation? Thanks again!

@MikeG it’s wordy but take a look at https://www.aopa.org/news-and-media/all-news/2013/may/flight-training-magazine/time-is-money


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What is the reason for the fence aft of the static ports on some airplanes and helicopters?
Airbus AS350 helicopters and Bell 206 helicopters have a 1/2" long fence behind the static port. CRJ 200/700 have a .060" step on the aft side of the static port. What would be the reason for these?

A clue, from a builders' forum:  "The static port can be tuned in situ. RV [builders] bond washers of varying thickness to get the desired calibration. This action mimics the more sophisticated milled "step" seen on some factory aircraft."

Do you have a photograph to illustrate your question? I found this image of a Bell 214 static port system which might suffice, although I'm not sure exactly what you're getting at.

Agree with @CatchAsCatchCan - image would be helpful.

In yet another example of unique helicopter design complexity, issues can arise when it comes to mounting static ports on the side of the fuselage where they may fall within the rotor wash. Ideally, static ports should be mounted in an area of undisturbed air, but other design considerations sometimes make this impossible. Additionally, helicopters can fly in directions other than forward, including in the direction of the static port itself. To counter pressure differences from sideslip, many helicopters use a second port on the opposite side. To adjust for rotor wash interference and ensure that the instruments read correctly in climbs and descents, small structures may be placed around static ports to speed up air flow with convex surfaces or slow them down with dams (what you refer to as fences or steps).
Although there is no picture provided, I think this may be what you are asking about:

For more information, you may look here:
https://books.google.com/books?id=rxryAgAAQBAJ&pg=PA409&lpg=PA409&dq=helicopter+static+port&source=bl&ots=p-H9klAkUl&sig=ACfU3U2Yda5qCljafTMg97TSlRH_EpC13g&hl=en&sa=X&ved=2ahUKEwj7paLgkLzqAhUIWa0KHZseD2cQ6AEwG3oECAgQAQ#v=onepage&q=helicopter%20static%20port&f=false


text:
Runway incursion episode/incident
Situation: I’m at the uncontrolled small airport (one runway and one taxi way).  The runway in the direction I will be taking requires backtaxing. I’m ready to taxi, I check the radio and get ‘clear and loud’ back.  I pull up to hold short lines, check the downwind, base and final, announce that I’ll be starting backtaxing and pull up to the runway (i.e. facing the final). To my horror I discover that there is an aircraft that must have just turn final. I execute 180 and exit the runway. The other aircraft lands and I re-enter the runway, backtaxi and take off. After I land I find a message from the owner of the flight school/club that own the place I used. He says that the airport called him, saying that I taxied in the line of approaching aircraft, but I couldn’t be reached on the radio. In fact, I did not hear anything during the entire episode (either from the airport, or the approaching aircraft). During my flight (1.7 hr) I used the radio repeatedly without any problems.
I am, of course, hugely embarrassed about the whole episode/incident. I’m a new pilot (130 hours) and I love the place where I rent the planes. I haven’t yet heard from the owner of this school/club (other than the initial voicemail), but I wonder if you could help me to answer the following: (a). what likely legal consequences will I face and (b) even if I don’t face any legal  consequences, is it likely that I will be banned from renting this school/club planes in the future.

Part (B) of your question is purely opinion based and is not a good fit for [aviation.se].

The b. question is about a common practices of the aviation clubs. I'm surprised by your comment.

Unfortunately, the common practices of a flying club are not common. It is based on the flying club and their opinions. People make mistakes. Most clubs will understand that until you start to affect their insurance rates. Now, if your Aeronautical Decision Making is habitually and consistently questionable or absent, that will be grounds for ending your rental rights.

You should report this incident in accordance with local regulations.

Would you please clarify who you got the radio check with?  And who is “the airport” that was trying to warn you on the radio?  Is there a FSS at the field?  And was the radio check made with the same entity that was attempting to contact you during the incident?

I said "...radio check" while tuned in to the local CTAF frequency (on COM1). I got a confirmation from either another aircraft, or the FBO (i.e. the 'airport'): "loud and clear". I did that after turning on avionics and obtaining the ATIS first (I used the other radio - COM2 - to dial that frequency). I think that the entity that tried to contact me was that same FBO (but, again, I'm not sure if those were the people who confirmed my original communication). FSS is normally contacted in the air at this location.

Dean F already made a good answer to this Q, I'll just pop my comments here: you did well. You used the radio, there might have been a misconception about frequency, or some other issue which prevented comms. Your intentions were good. And, you did spot the other aircraft, vacated runway and saved the day. What I would do, is I would definitely report this incident. Reports are used to primarily advance safety, not to find and punish the "guilty". Once again: you lived to tell the tale, tell it. God knows why there almost was an accident, but you prevented it. Learn from this and all is ok.

From your description, you did nothing wrong. But, there is a possibility that one of you was on the wrong frequency. It is hard to prove that unless the radio calls were recorded. There is iPad software for recording your radio transmissions.
The more important lesson to be learned is that a nontowered  field is supposed to be pilot-controlled. But, it is perfectly legal to fly in pilot-controlled airspace without talking on the radio. Even if neither of you made a radio call, you would be within the FAA regulations.
The mistake that was made was that neither of you maintained situational awareness. You should always visually clear the traffic pattern while taxiing to and holding short of the runway. This is especially true for final approach. In the case of a high-wing aircraft, you will have to angle the nose of your aircraft downwind toward final. Otherwise, your wing will block your view of final.
P.S. It might not be a bad idea to file a NASA report.


text:
Which FAA knowledge tests share question database (e.g. Commercial and Advance Ground Instructor)?
I'm curious if there is an overlap between tests. I'll be on instrument, commercial, ground instructor, cfi and cfii ratings in sequence and I wonder if I could take similar exams back to back. Thanks

If you study for the Instrument written you should not need any additional prep for the Instrument Ground Instructor. The Advanced Ground Instructor is similar to private and commercial but also includes questions about other aircraft e.g. rotorcraft, multi-engine, glider, baloons, and I believe sport. It may have questions about drones but I don’t remember. Here are some sample questions. https://www.faa.gov/sites/faa.gov/files/training_testing/testing/test_questions/agi_questions.pdf


text:
Finding published holds information
A friend of mine was on a IFR flight plan in central Utah. As he was approaching Milford VOR (MLF) he was told to enter hold ("Cessna N12345, hold south of MLF VOR AS PUBLISHED)". There was no published hold depicted on either paper chart, or that in ForeFlight. However, I found out that if you try to enter hold in FF like any unpublished hold (touch the fix and select hold) there are 2 published holds to that VOR. Question: where is that hold published (other than in FF)?

unpublished holds -- https://www.ifr-magazine.com/uncategorized/unpublished-holds/ -- might give some context for non-IFR readers.  Obviously not an answer.

You can always ask ATC where it's published!

Holding information is held as an 8260-2 form. Here is the one for MLF:
https://nfdc.faa.gov/webContent/content8260/UT_MILFORD_VORTAC_REV17.pdf
As you can see, there are two holds established at this VORTAC, one to the north (R-007) and one to the south (R-168). This is what ForeFlight has as data.
It also indicates which pattern is used for which procedure. In this case, the south pattern is used in the KMLF textual departure procedure. Specifically:
DEPARTURE PROCEDURE:
Rwy 16, climb direct MLF VORTAC, thence...
...aircraft departing on V235 or V21 southbound proceed on course.
...all other aircraft climb to 11000 on MLF VORTAC R-168, then right turn direct MLF VORTAC. Continue climb-in-hold, right turns, 348° inbound to cross MLF VORTAC at or above MEA/MCA for route of flight.
I think it’s a bit unfair to your friend since this hold is not actually published on a chart.

Thank you for this information. So, it is a rather obscure source of information. I'm glad that FF accesses it, but, you're right, it is a bit unfair of ATC to assume that it is a readily accessible information.


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How does a bearingless rotor work on a helicopter?
I have read about some helicopters now using a mechanism referred to as a "bearingless rotor". From what I've seen online, the system involves a flapping hinge.
What I struggle to get my head around is how the rotor spins without a bearing. How is this achieved?

There are still bearings for the spinning part of the rotor mast coming out of the transmission, and for blade pitch in the hub itself.  The bearingless part is that the blade flapping and lead/lag functions are accommodated by "elastomeric" hinges, basically, rubber-type compliant bushings.  It's very similar to the control arm attachments on modern cars where the control arm is attached to the frame with a large rubber bushing, bonded into a sleeve that is bolted to the structure, that accommodates the arm's rotation while isolating the control arm from the car.
Some helicopters also achieve this function by making the root section of the blade into a flexible shaft that is free bend in the flapping and lead/lag direction.
Hingeless rotors have been around since at least the late 70s.

Are you saying that "hingeless" and "bearingless" rotors are the same thing?

I would say so.  The motions at issue are the lead/lag flapping movements, which if they are mechanical, are hinges with bearings, or if using elastomeric bushings, don't and you wouldn't consider an elastomeric mounting a "hinge" in the mechanical sense. When I first read about this innovation in the late 70s the term was "hingeless rotor", as opposed to "rigid rotor" like the Cheyenne had.  Another term for them was "semi-rigid" rotors.

Just wanted to clarify since the question is about a "bearingless rotor" and you referenced "hingeless rotors" in your last sentence.

No that is wrong. There is a subtile but clear difference between hingeless and bearingless. Sophit points that out too.

First of all a short clarification about the difference between bearingless and hingeless rotors is given followed by the description of the "bearing-part" of a rotor.

Without entering in the detail why the blade has to have the possibility to 1)flap, 2)lead-leg and 3)pitch in respect to the hub, bearingless, hingeless and fully-articolated rotors are different technological and historical ways to get these 3 movement. In the fully-articolated rotors these 3 movements happen around 3 bearings. The use of bearings was the first logical and technological way to let those movements free and virtually all of the oldest helicopters used it. Typical example is the Sikorsky CH-53 and this is also a very nice example about how much the rotor head can become packed:

CH-53 Main-Rotor head. Source: https://i.pinimg.com/originals/21/6c/82/216c8236203d4906840a821063aef3a8.jpg
Here I highlighted the axis of the 3 bearings for flapping (green), lead-lag (blue) and pitch (red). Hopefully they can be recognised despite the perspective. Note that in this particular design yet another hinge is present, i.e. for blade's folding, in yellow. Circled in yellow are the hydraulic actuators for folding.
Unfortunately bearings were delicate and had to be greased and substituted quite often with high costs. The second iteration saw therefore the replacement of the most stressed bearings (the ones for flapping and lead-leg) with a blade whose root was soft enough to let these 2 movements happen. The bearing for pitch changing survived. These were/are the hingeless rotors. The first rotor of this type was the one of the BO105. The use of glass fiber instead of aluminium for the construction of the blade was surely a key technological aspect for the success of this kind of rotors.

BO-105 main-rotor head. Source: https://www.flyingbulls.at/fileadmin/_processed_/6/1/csm_bo-105-19_9ad787a50b.jpg

BO-105 main-rotor head dismantled. Source: https://aviationweek.blob.core.windows.net/aviationweek19670626thumbnails/Media/0x600/1967062683_1.jpg
These 2 pictures clearly show how much cleaner this kind of rotor head was/is. The hinges for flapping and lead-lag are just gone replaced by a softer blade's root (circled in yellow). The hinge for pitching (circled in red) has survived and it just inserts inside the X-shaped one-piece titanium rotor head. The mutual movement is assured by a set of 2 bearings, not visible in the picture.
The last iteration was obviously the elimination of the pitch bearing too, giving a bearingless design. Here the 3 hinges are replaced either by a blade's root soft in all 3 directions or by elastomeric hinge(s). Typical examples are the H135 and the AS350 respectively. Also in this case the switch to CFRP as building material for the blade has been a key enabler together with the technological maturity of elastomeric materials.

Bearingless rotor head. Source:
https://assets.verticalmag.com/wp-content/uploads/2019/03/Blades-cross-section-1024x633.jpg
This picture shows the main rotor head of an H145-D3(?). Here all 3 hinges are gone replaced by a soft CFRP blade's root, in brown in the cutaway.

That's for the connection blades-rotor head. As pointed out by @John K, there are still bearings for the spinning part of the rotor. The rotor head with its shaft is inserted in a gearbox which is used to mechanically connect the rotor to the engine(s) and to the fuselage. A typical gearbox looks like the one in this picture:

Main gearbox of the Sikorsky S-49. Source: https://www.sikorskyarchives.com/S-49.php
Even if this gearbox dates back to the 40s, its layout is still up-to-date. In this cutaway is easy to see that everything which rotates does it thanks to one ore more sets of bearings (I circled a couple in blue). So whatever the connection blades-hub is (hingeless or bearingless), the gearbox, together with any other rotating mechanical parts, is anyway "bearingful"


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Is it possible to get into a spin situation while coordinated?
Is there any possibility of spinning while coordinated?  Of course, any CFI would tell you no, but I wonder if the the following situation could result in a spin while coordinated:

Slow level flight in a turn
Outside aileron needed to counter over-banking-tendency; rudder as needed to keep coordinated
One wing has a higher AOA due to aileron deflection
Pull back on elevators, stalling one wing first

You have to remember turbulence. Also even in a turn aileron is not deflected. Difficult to understand this question

At slow speeds, opposite aileron is needed to keep a constant bank.  I'll try to find something to back this up.

Yes, what you describe is perfectly possible. A stall in banked flight can result in a spin, given you fly the "right" plane.
My first flight in an ASW-20C was late in the afternoon, when most thermals had died down. I got a winch launch to maybe 350 m (the ASW-20 is rather poor in winch launches) and tried to find an updraft. In maybe 270 m I found one, but it was weak and narrow. So I slowed down to tighten the circle. Having flown a Discus all day before, I was not really alerted to watch out for a departure on stall. At some point the lateral stick force faded markedly, and out of the Discus habit I threw the stick into the circle so the inner aileron would deflect downwards to support the stalling wing. Bad choice! This aggravated the incipient stall, the aircraft rolled into the circle and pitched down. While the view from the cockpit before was blue above and green below, it got now all green. I was entering a spin.
The weakening stick force was the result of flow separation over the aileron, and while the Discus airfoil would still provide more lift with more deflection, the ASW-20 airfoil simply went into full stall.
I immediately pushed down and levelled the plane, and went on to land straight away, having lost maybe 100 m. It all took only seconds, and only long after the flight I could piece all details together. My next flight in the ASW-20 was when the thermals were much stronger. I went up to 1000 m and did stall training until I knew how it felt when stall starts and what my options are. I should had done this on the first flight, and do so on all subsequent new types I fly.

outside aileron to counter over banking tendency

One of the best things to practice at altitude, especially in a new type of aircraft, is its tolerance to "opposite" aileron to control "overbanking tendency" in slow flight.
This is generally not the best technique because the inside aileron is down, raising the angle of attack. If a stall occurs, it will be on the inside wing first.
There are other ways of "roll control" starting first with the vertical stabilizer and wing dihedral.  Rather than using opposite aileron (each aileron is half a speed brake), try slipping a bit and let the vertical stabilizer and wing dihedral help.
By the way, rudder into the turn also helps one roll away from the turn, again, because the vertical stabilizer is asymmetric to the center of gravity.  Any yawing force it creates also produces an opposite rolling force.
But if rudder is over done, relative wind is now from the  outside, and dihedral will try to roll you into the turn.  Aircraft with very long wings and small tails may be more susceptible to this and may benefit more from slipping.
Seek expert advice from someone familiar with the characteristics of the particular aircraft. You may find that few recommend opposite aileron to "hold up" a turn.

Aeroplanes with no wing washout (twist in the wing that gives a lower incidence outboard) can drop a wing more easily while ‘holding off’ bank when flying close to the stall.
When turning, the wing on the outside of the turn is going faster and so develops more lift.
All the powered light aircraft I can think of need some opposite aileron to hold the bank angle steady when turning.
Let’s say we are in a 30 degree banked left turn 10 knots above stall speed in a plane with no washout. We need some right stick/yoke to keep the bank angle steady. Right stick means the right aileron is deflected upwards and the left aileron is deflected downwards.
The downward deflected aileron (left in this case) increases the effective angle of attack on the left outboard wing putting it very close to the stalling angle.
Any slight inattention in flying, a fraction more nose up for example or a gust could result in the left outboard wing stalling and dropping.
One should apply the correct control inputs to recover from an incipient spin at that point, but mere mortals with lack of recent training will more than likely apply more right aileron which will make the situation much worse.
While giving Britten-Norman Islander (BN2) training for a type-rating to a young man this happened to us. The aircraft entered a fully developed spin but recovered with standard actions by the startled instructor.
While pondering the incident later I realised what had probably killed a few of my friends flying in the tropical mountains in Islanders with heavy loads and without the benefit of 5000 feet of height agl.
I am not disparaging the Islander, while I don’t know any pilot who really loved flying them, they are a good bush plane and we were provoking it in the above case. The stall warning was on and the student was just that bit slow in relaxing the back pressure etc.

Somewhere in my initial training almost 50 years ago, I seem to remember that in a gliding, descending turn you need to ‘hold on’ aileron to keep a constant bank. Maybe others could help here.


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Does the payload of an ICBM or regular airplane bomber explode if the ICBM or bomber is shot down?
As the top says, does the payload of an ICBM or regular airplane bomber (please excuse my ignorance) explode if the ICBM or bomber is shot down? So, say a defense laser in space (yes, I know; this is hypothetical for a setting, but) disables an ICBM in mid-flight, would the bomb detonate as well? If I recall correctly, even regular bombs shouldn't explode if the bomber explodes, that is, the bombs are meant to go off exactly when the bombardiers wish them to go off. Thanks.

It happens enough that there's a term for it, Broken Arrow. Don't look into it if you want to sleep at night...

@RonBeyer agreed.... this message will self-destruct in 2 minutes

Usually it won't set off the payload.
The payload is designed in a way to prevent it from going off prematurely - say it needs to be armed before it can go boom. Usually you would only arm the trigger mechanism in the end phase - directly before payload delivery since you want to hit your target and not your own installations.
Modern explosives are remarkably unwilling to go off without the correct primer. Some of them will not explode even when set on fire (and just burn in that case). However some others might go off if they just get heated enough.
For nuclear weapons, it's even more difficult to determine. If the conventional explosives go off in an unpredicted pattern, you might have a conventional explosion but not a nuclear. If these explosives do not go off in the right pattern, the bomb might never reach criticality and fission (and possibly fusion) stage of the weapon will never trigger. In any case, wherever it went of, the bomb might scatter plutonium, uranium and other radioactive material where it was shot down.
That said, an aircraft or missile crashing can cause ordnance to arm and there was at least one case where a US bomber with live nukes crashed, the bombs partially got armed on crash but didn't trigger in the end. https://en.wikipedia.org/wiki/1961_Goldsboro_B-52_crash

Nuclear/atomic bombs and warheads both have elaborate automatic arming mechanisms built into them that have to detect the correct sequence of (for example) acceleration, ambient pressure reduction, free fall, re-entry heat, and g-force in order to enable the firing mechanism for the warhead. If any of these signals is missing or presents itself in the wrong order, the arming mechanism is defeated and the device is not supposed to explode.


text:
Weapon vs normal aircraft track
I've read that some radars can detect stealth aircraft or aircraft with low radar cross section but are not powerful enough to allow a "weapons" track or allow a missile or another aircraft to lock on to it.
What is the difference between the 2 radar tracks? Is one less accurate in terms of pinpointing the aircraft range by a few meters or kms?

Not necessarily a matter of power; a low frequency radar may be able to determine that a stealth aircraft is "out there" somewhere, but with such low resolution that its presence is about all that's known - location precision being on the order of counties or small states. More power won't help; higher frequency, thus shorter wavelengths, makes for greater precision, but that's where stealth works best.

@RalphJ -- looks like the above comment could be grounds for a good answer

@quietflyer Actually I think Carlo's answer says everything that needs saying with regard to low frequency radars.  Just adding that it's more about frequency than about power isn't much of an answer in & of itself, thus my comment rather than answer.

Virtually any radar in the world can detect and track a stealth aircraft as well as to provide a “kill chain” to direct and guide weapons at it.  The key is at exactly what range they can do this and that’s a function of the radar cross section of the target.  Stealth aircraft typically, depending on the bearing and azimuth a radar illuminates them from have an RCS on the order of a BB pellet; a conventional aircraft eg a SU-27 has an RCS around the size of a two story house.  Certain kinds of radar sets like low frequency radars can detect stealth aircraft much better than X-band radars can but are inferior for miniaturization or for providing weapons guidance, limiting their effective ranges.

Depending on material, a BB pellet may be totally invisible for radar

The range is also dependent on the size of the radar antenna. The AIM-120 missile radar antenna has to fit in a 7" diameter space. The missile flies to an intercept location data-linked from the shooter's aircraft which has a much larger radar. The missile radar goes active in the final stage at close range.

"or for providing weapons guidance". May I know how so. You are able to detect but not point it's location?

So does it mean that virtually any radar that can detect stealth will flag that anomaly in their system, even out of their target range? Some documentaries depict that a normal radar won't flag an alert but behave oblivious to the threat. Please see this video for an example("Why the F-22 Raptor Still Reigns Supreme") https://youtu.be/jvNN_ehD_0M


text:
How do Luneberg lenses hide a stealth aircraft's real signature?
I thought aircraft was supposed to be stealth and not show up on radar under the right conditions. This article says

Removable Luneburg lens type radar reflectors are sometimes attached
to military aircraft in order to make stealth aircraft visible during
training operations, or to conceal their true radar signature.

So even with Luneberg lenses, will there be 2 blips on the radar screen, one from the reflector, and a weaker blip from the aircraft signature?
If I am potential adversary observing training involving stealth aircraft, is it possible to filter data around the "visible aircraft" to get its true signature? How is this difficult?

I’m pretty sure “conceal” here means to make the aircraft’s radar signature much worse during non-combat operations, so enemies won’t know how much smaller it will be during real combat.

Since the attached reflector and the rest of the aircraft are at the same position and velocity, they form a single target. There will be no second blip. They're one and the same.
When a target has moving parts, like exposed engine blades, they distort the signal, producing a radar signature. This signature will be obscured by the much stronger reflection from the Luneberg lens.
This is a small side effect of the attached reflector's purpose: to allow allies to relatively easily track and target the aircraft. However, such a handicap also keeps them from experiencing stealth as it is and developing tactics to deal with it.
Even when high-res radar imaging is used, spatial separation alone is generally not precise enough to separate the signatures of other airframe parts from the removable reflector on its own. Engine signatures (JEM) can be separated through Doppler shift. When the engines are concealed, there's not much in the way of distinguishing the airframe's static signature from the reflector's.
For more technical information, look into the subject of NCTR (non-cooperative target recognition).
Some ACARS papers can be easy to digest.
Pages 177-184 (28-1) make for a good starting points on the basics, 97-110 (8-3) address reflectors and their effect on the radar signature more specifically.

My impression is that Lunberg lenses or reflectors are used only during training to easily allow all participants to track the aircraft easily. It's not supposed to be used during actual combat?

It's definitely not meant to be used during combat.

There is no "signature" that identifies an aircraft by name.  There is only a radar reflection.  In most cases, a radar cannot tell the difference between an F-35 and a suitably sized, supersonic toaster.
Let's assume the radar uses a radio frequency of 3 to 11 GHz.  Thanks to the wave nature of radio waves, the measurement will be limited to around 20 cm resolution.  That's in the direction of the radar beam; in the other directions it will be a lot worse.  Getting that ~20 cm resolution is going to require some expensive electronics, but militaries can afford it.
The Luneberg lens is only about 2 cm from the aircraft, so the chances of finding the aircraft's radar cross section from a direct measurement are very small.
Even if you could distinguish the radar reflection of the lens from the radar reflection of the aircraft, your radar can easily be defeated by adding a few more lenses.
In practice, a military would find it much more convenient to use Google Image Search to find some pictures of the aircraft.  Then they use a supercomputer to estimate the radar cross section based on the shape.
If it's not cloudy, you could skip the radar and just use a bunch of binoculars.


text:
Is delta wing design on a fighter a compromise?
I apologize for not providing a reference, but I recall reading a Russian designer, I think he had something to do with Mig-29, while talking about Mirage 2000, said that a delta wing is essentially a compromise. He mentioned some technical challenge faced by the designers and if they cannot overcome they fall for a delta. It overcomes something but leaves something else to be desired.
Can somebody please shed some light on it? I am just an enthusiast.
One thing is for sure, no US fighter is a delta.

I think it's safe to say that any design feature on an aircraft is a compromise.

@PeterSchilling: Any engineering decision is a compromise, especially when you include money in the equation.


text:
Is landing the 'wrong way' possible on an aircraft carrier?
I wonder if the arresting wires would work the 'other' way?

Theoretically? Sure. But you're not going to like it! You'd be landing downwind at a much higher speed relative to the carrier deck, and there aren't arresting wires at the end you'd be touching down on so you're on your own for stopping. If you make it to the wires at the end and the arresting hook engages with them it could bring you to a stop, but you will probably have departed the deck when it does so that's probably not going to be a fun experience...

The pulley in the deck looks like it's designed to work in the normal range of angles for the wire only. So the wire would probably not unwind normally even if you caught it (in addition to it being too far so you wouldn't have the distance to stop anyway).

Not to mention the complete lack of landing guidance going in the wrong direction, which will make touchdown hard.

IIRC, a high wind speed + full speed on an aircraft carrier can match the stall speed of some light sport aircraft. In that case, they could land any way they wanted—even perpendicular!

WWII's Essex class was designed with a high reverse speed in order to allow such operation, and I found a citation for the CV-3 Saratoga (different class) briefly operating backwards in 1945 due to kamikaze damage suffered at Iwo Jima.

The lights are located at the aft of the ship. Without lights you would have no idea whether you were high or low.

Does the carrier have a ski-jump?

You're going to run into a few issues.
Below is a diagram of a typical aircraft carrier layout. Normally aircraft would land from right to left. The four cables are located closer to the right end than the left. The extra length after the cables is both to allow a "bolter" (YouTube) time to accelerate in case of missing the arresting hook; and to allow an aircraft that caught a cable time to decelerate.
Notice how far an aircraft can continue down the deck (YouTube) after catching the cable. Even if you catch the first cable, you might go off the aft end of the ship.

Another issue is the way that the arrestor cables work. The housing where the cable enters the flight deck is curved to allow the landing aircraft to pull on the cable as it decelerates. The cable is not designed to be pulled in the opposite direction, which could cause it to break (YouTube).

Aside from these issues, you would also lack the FLOLS system that provides physical guidance during the landing. And as voretaq7 also mentioned, you have to either touch down late, or catch the cable late, both of which could pose problems.

That 2nd video is an amazing insight into aircraft carrier ops - thanks! 3rd one's nasty, gave me shivers.

You'd need to touchdown early and basically taxi into the 1-wire (the real 4-wire) to ensure you didn't drop off the back of the boat.  Also, shore based arresting gear is typically bi-directional, so I'm not sure if the arresting gear engines could handle it or not.  My gut is to say that something very bad would happen, but if shore based systems can handle it, I assume the carrier based systems can too.  In fact, if you lose your brakes on the carrier one of your options is to taxi back and snag a wire if you're able; otherwise, you literally try to crash into something that will stop you.

@RhinoDriver Does shore-based arresting gear look like in the picture (where the cable housing is curved in one direction), or is it symmetric?

@cpast Honestly, I've never paid much attention to it. I assume its probably symmetrically shaped. However, most of this is pure conjecture because there is a 0% chance the boss is gonna let you land the opposite direction anyway.

@cpast On land, the gear is designed to work both ways, because it doubles as overrun protection. Example

I'm not an expert, but the location of the catch wires on that drawing looks significantly out of kilter - far to close to the bow. Though it also shows 4 elevators and 4 catapults, so I guess it's not exactly a Nimitz-class, just a generalization.

@fooot Yep, +1. The real question is whether or not the cable, or engine, would be able to handle a reverse engagement. Without a whole lot of knowledge on the subject, one would at least assume, on the surface, that the hydraulic engine is simply providing resistance as the cable pays out. The engine should provide the hydraulic braking, as either way the cable is lengthening, but perhaps one direction is more efficient etc, on the carrier based systems since they are undoubtedly designed with only a unidirectional engagement in mind.

Fixed wing aircraft will never land the wrong way on the boat.  However, I've done it many times in the H-3.  It's a difficult landing even in a helo, the ship is closing on you and you have a tailwind.  The approach becomes excessively steep very quickly.


text:
What is Vg in this VG diagram?
There are two vertical bars beyond Vne labelled: @800#-Vg = 150MPH and @840#-Vg = 147MPH. These lines represent Vg at two different weights.
Can you tell me what  Vg is, where it's officially defined, and how it's calculated?

Obviously not Vglide

It is the maximum speed in gusty weather. Aircraft designers and certification authorities make assumptions about the maximum vertical gust an aircraft will encounter and design the strength of the wing spar for it. Here 24 feet per second are assumed.
Depending on the mass of all parts not creating lift (fuselage, vertical tail, pilots, equipment ...) the load on the wing spar varies with the aircraft mass. For that reason, lighter aircraft can afford to enter a gust at a slightly higher speed. Here, the 800 lbs glider may fly up to 150 MPH, while the 40 lbs heavier glider should stay below 147 MPH to suffer the same wing spar bending loads when flying into this 24 fps gust.
Flying faster means flying at a higher dynamic pressure and at a lower angle of attack. Entering a gust will increase the angle of attack by the ratio between gust speed and flight speed. Since dynamic pressure is proportional to the square of flight speed, a linear wing lift increase due to higher flight speed will remain when entering a vertical gust. Also, at lower speed the angle of attack might be high enough that the angle of attack increase due to the gust will stall the wing, putting a natural limit on the load increase. Flying faster will remove this load limitation.
The maximum flight speed in gusty weather v$_g$ is a limit speed just like v$_{NE}$, but applies only in specific weather conditions (admittedly those which are most suitable for glider flying). While v$_{NE}$ might be determined by a minimum flutter margin, a formula in the certification requirements or structural loads in maneuvers, v$_g$ is determined by the maximum load factor the aircraft can tolerate.

Is this a structural limitation rather than aerodynamic, like Vne? How would someone use Vg in practice?

Yeah, I'm unclear on how this relates to Vne, especially since the 800lb Vne is is 7.5mph slower than the 800lb Vg.

At the end of para 2, I believe you mean 24 fps, not 24 fpm. Either that, or you meant minute at the end of the first para, not second.

@FreeMan: Thanks, yes, it is fps. A 24 fpm gust would be a boring affair.

@egid: Yes, makes me wonder myself, since v$_{NE}$ is the maximum speed in calm air. I guess this is to show where the theoretical v$g$ is, but v${NE}$ bites first. I am more used to v$_B$ for this limit, but when you look at the v-n diagram, v$_g$ is where v$_B$ should be.

@PeterKämpf can you address my question

@rbp: I added a paragraph as a response, but to me the distinction between structural and aerodynamic limitations is not clear. If aerodynamic loads are too high, you exceed structural limitations, so it is both.


text:
Can I use an expired IFR-certified GPS with an out of date database in lieu of DME?
I know I should update the database in my GPS, but nevertheless I'm wondering if i can use an expired IFR-certified GPS with an out-of-date database in lieu of DME

This is one of those where the answer is probably "Yes, you can, but that doesn't mean you SHOULD."

The answer would appear to be "Maybe", per the AIM (TBL 1-1-6): You are permitted to use an IFR-approved GPS "In Lieu of ADF and/or DME", however the footnote (3) to that table tells us that this use

Requires current database or verification that the procedure has not been amended since the expiration of the database.

So if you're using the GPS to calculate something like "5 Miles DME from JFK VOR" that would seem to be acceptable (the FAA is unlikely to pick up JFK VOR and move it without making a lot of noise about it), but if you're using by punching in a fix that's defined by DME and a radial you would need to somehow verify that the fix hadn't been redefined.
You can accomplish that verification in a number of ways (a current IFR chart on your iPad or a current paper chart seem like obvious ways). As long as you can satisfy yourself (and the FAA) that the data in the GPS is accurate the operation would appear to be permissible.
It would also appear that if you verify that the fix has been redefined (say FUBAR was 5 DME, now it's 6 DME) you could still use the GPS distance data in lieu of DME, just not the incorrectly-programmed fix. That is, you could determine when you're 6 miles DME from JFK VOR, and since you know the FAA didn't move the VOR and that the fix is now at 6 miles you now you're where you want to be.

I would argue that the answer is currently a definitive yes as long as you are not executing an IFR approach or any published procedure including RNAV operations.
It's a case of permissibility by omission as no language exists which prohibits this operation.
The AIM contains this reference.

1-1-17 Global Positioning System (GPS)
b. Operational Use of GPS
2. IFR Use of GPS (b)
3. All approach procedures to be flown must be retrievable from the current airborne navigation database [...]

RNAV operations are mentioned in AC 90-100.

AC 90-100, U.S. TERMINAL AND EN ROUTE AREA NAVIGATION (RNAV) OPERATIONS, paragraph 8a(3): The onboard navigation data must be current and appropriate for the region of intended operation and must include the navigation aids, waypoints, and relevant coded terminal airspace procedures for the departure, arrival, and alternate airfields.

I also found it interesting as I was renewing my CFI certificate that my training course made this assertion, but provided no citation.

Although you can fly en route IFR with an expired database, approaches are not authorized.

It's also interesting to note that Garmin says you can still file /G with an expired database.

You may file your flight plan as /G if your 400W-series unit is an authorized IFR installation. The 400W series is a TSO C146a Gamma-3 (Class 3) authorized GPS navigator. If you are flying en route, you may file /G with an expired database only after you have verified all route waypoints. Approaches may not be flown with an expired database. See your approved Airplane Flight Manual Supplement for more information.

IFR Refresher also agrees.

While it is not legal for approaches,
you can use an expired database for en
route and terminal operations, as long
as you verify the data is still correct,
generally by reference to paper charts.

The question of whether or not GPS can be used in lieu of DME has been answered. Combining the two I draw the conclusion that the answer is yes.

@ymb1 Thanks for the edit!

@CharlesBretana An autopilot would be coupled to something installed in the aircraft (like an FMS or an IFR-certified GPS) that has a database in it. Not to an EFB. And what the FMS or GPS refers to is that database, not an approach chart (or plate), which is designed for human viewing, not driving  an HSI or Navigation Display or coupled autopilot. The data underlying both the FMS database and the published approach chart will be the same, but the idea of "coupling an autopilot to an EFB" seems like a mismatch.


text:
What's the right way to take a dog in a small plane?
I would like to take my friend and his dog for a flight. What's the safest way to take the dog? and what's a good way to protect the dog's hearing?

It is not possible for this question to be complete without referencing this - https://www.youtube.com/watch?v=SN77b9DqEbc

Someone needs to float a cat as well ;-)

My wife and I have a small dog that frequently travels with us.  We place her in a soft-sided kennel that straps onto the seat belt.  While this doesn't do anything to help in a sudden impact (i.e. crash), it does help to keep her from being bumped around in case of turbulence.  For larger dogs, I know that there are seat belt tethers out there.  My suggestion would be to strap them to a rear seat.  In either case, the big issue is to ensure they don't get in the way while operating the aircraft.
As for the hearing part, a company called Safe and Sound Pets makes a product called Mutt Muffs.  Their website is here.  My wife and I bought a pair, but our dog wouldn't have anything to do with them and they were returned.  Of course, your dog may be better behaved :D

When you are flying with pets, there are certain things you need to consider which aren't normally considered when flying with people.
Depending on the size and weight of a pet, a suitable carrier in which they can fit in is needed. You also need to make sure that this carrier must be able to fit in the plane and be secured. If the pet is large and cannot fit in carrier which can in the plane, a pet harness that can be secured to seat belts or seat belt attachment points can also be considered. It should be an approved car safety transport harness.
Some general guidelines are:

The pet should be secured.
Allow the animals to relieve themselves before being loaded.
As soon as the trip is complete provide water and some nutrition for the animals.
It is very important to always clear the seats and plane after the journey. Dogs are very susceptible to parvovirus.
Animals do not know that swallowing and yawning will help their ears in altitude changes like the humans. Hence, it is better the minimize the rate of climbs and descents to minimize discomfort to animals.

AOPA has an excellent and detailed article about flying with pets. For the hearing issue, they state:

... [by putting cotton in ears, it can] stuck in the wax of the ears
  and cause problems. The better option is to use pillow foam instead of
  cotton. Pillow foam is similar to what earplugs are made out of and is
  not as likely to get stuck in the ears.

I hope that your journey won't go like this:

image source

My dog flies with his Mutt Muffs.
In a pinch, you can also put cotton balls in a dogs ears.  Carefully put them in enough that he cannot paw them out, but not so far as to get into the sensitive parts.


text:
Are there any true aerobatic helicopters that aren’t built around the Bo-105?
I know that some aerobatic maneuvers have been performed (legally or otherwise) in many different models of helicopters. However, to my knowledge, I have only heard of Bo-105s being specifically modified for this purpose, supposedly due to their hingeless rotor system. Are there any other aerobatic models flying at air shows in other parts of the world?

The AH64 will loop, roll, and hammerhead.


text:
How is notification of UAV flight handled in an area with multiple heliports?
I'm trying to get up to speed on drone licensing and regulation. I'm in central Houston, well away from either of our major airports; but there are literally 50 heliports listed on B4UFLY as being in my immediate area. AirMap shows 32 of them actually overlapping my current position. Houston did just get the brand-new shiny "Low Altitude Authorization and Notification Capability", but of course only for the two major airports.
I'm absolutely not interested in doing anything wrong; but does this mean I can't legally fly a drone in circles around my back yard at eye level without calling 32 different helicopter operators to let them know? If I managed to find a nearby spot where only, say, four or five areas were overlapping, could I actually call them and notify them, and would that make me properly compliant?
Answers relating either to Section 336 or Part 107 are appreciated. At the moment I'm just a hobbyist; but I do plan to get a remote pilot certification, because my ultimate use is academic research, which falls under commercial rules. Thanks!

Tl;dr: Yes, you are required to contact all airports and heliports within a 5-mile radius.
Unless you have a part 107 certificate you are operating under the rules of 14 CFR Part 101, Subpart E - Special Rule for Model Aircraft. Since the FAA is generally prohibited from setting rules for hobbyist operations, the only specific regulations are under this subpart. They are frustratingly unclear and refer most rulemaking to undefined “nationwide community-based organizations.” Although which organizations, or even what such an organization consists of, are not defined, the FAA works closely with the Academy of Model Aeronautics which publishes a handbook for safe operation.
One of the few specific regulations subpart E does include is the following.

14 CFR § 101.41 (e) When flown within 5 miles of an airport, the operator of the aircraft provides the airport operator and the airport air traffic control tower (when an air traffic facility is located at the airport) with prior notice of the operation.

This means that to be in compliance you must notify all 32 heliports in your area. Note that it does not say you must get permission, only that you must provide prior notice. It also doesn’t say what kind of notice, nor how far in advance, nor if you must notify them every time you fly or if it can be an ongoing situation.
Mongo’s suggestion of mailing a letter to each one would be a good approach. Make sure you keep a copy of the letter sent and a record of who you sent them to. That way if a problem arises you have documentation that you did make the required notification.
Beyond notification, there is no mention of airspace classes, flight restrictions or anything else. It only specifies that:

14 CFR § 101.41 (d) The aircraft is operated in a manner that does not interfere with and gives way to any manned aircraft

That’s not to say that you don’t have to follow flight restrictions. That would fall under the elastic clause:

14 CFR § 101.43 Endangering the safety of the National Airspace System.
  No person may operate model aircraft so as to endanger the safety of the national airspace system.

If you are planning to fly for research then you should certainly get a part 107 suas pilot certificate. Part 107 regulations are much more well-defined. If operating under part 107 there is no requirement to notify the airports and heliports. An suas license requires you to know how to read charts and know what type of airspace you are operating in. By default part 107 is restricted to uncontrolled airspace (class G). To operate in controlled airspace you must apply for a waiver and follow the rules of the waiver. Once you have a waiver for controlled airspace the requirements for getting permission depend on the specific airspace you need to fly in. Under part 107, in class G airspace there is no requirement to notify anyone.

This is all fantastic; thanks very much. It's not technically any different than what I've already read for myself, but the context and experience is everything!

Simply put, and barring any other airspace issues, you will need to contact each of the airports.
You decision to go for a 107 certificate is a good move, because there are many complexities operating in an urban setting.
If you have 32 heliports impacting your back yard operation, I would contact each one.  It may seem burdensome, but an approach I have taken is to mail a letter to each airport manager, informing them of where and generally when I expect to operate.  I usually specify that this will be ongoing, and include my contact information, and the location(s) I expect to operate from indicated on an included map.
As for eye-level, it doesn't matter.  As soon as you leave the blade of grass, your device falls under the FAA domain.  If you are learning and therefore conducting activities under 101, you might document that in a flight log, in advance of your flight(s).

This FAA link seems to say that up to 400 feet above the ground, (or 400 feet from a "structure") is perfectly acceptable.

If you are flying under part 101, then you will need to notify each airport within 5 miles.  You will find this in the FAA UAS FAQs.  Flying is any activity conducted above the surface...not 400 feet.  So once you leave the blade of grass, you are flying.

@abelenky You’re both right in different ways. The 400 feet is for part 107 pilots. The “contact every stinking hospital heliport in the continental US” is for section 336. Once you have a part 107 license you aren’t required to contact everyone. You follow the airspace rules and any waivers you have and request ATC clearance through the website.

The idea of a one-time blanket notification is a huge help; if that's legit, then I don't mind doing it at all. It was the idea of technically having to call up 32 different entities every time I took off that was making me think I would never be able to fly (as a hobbyist) anywhere near my house.

I have done that, and I have found no regulatory prohibition of an ongoing notification. I did discuss it with an FSDO inspector who handles UAS enforcement, and he found no problem with it. However that is not a letter of interpretation. He suggested rather than a one-time phone call, to do so in a hard copy letter, and I adopted his suggestion.  When traveling I sometimes still use a phone call and say that this is for multiple instances.